sync: add OwnedSemaphorePermit::semaphore (#5618)

tokio/src/sync/semaphore.rs

@@ -676,6 +676,11 @@ impl OwnedSemaphorePermit {
         self.permits += other.permits;
         other.permits = 0;
     }
+
+    /// Returns the [`Semaphore`] from which this permit was acquired.
+    pub fn semaphore(&self) -> &Arc<Semaphore> {
+        &self.sem
+    }
 }
 
 impl Drop for SemaphorePermit<'_> {